1. Field of the Invention
Embodiments of the invention relate generally to methods and apparatuses for controlling the timing used to sample signals produced in relation to data stored in a data storage medium such as a magnetic disk of a hard disk drive. More particularly, embodiments of the invention relate to methods and apparatuses for controlling the timing used to sample signals produced in relation to stored data based on a phase error calculated as a difference between an equalized signal and an estimated data signal, both generated in relation to the stored data.
2. Description of Related Art
A hard disk drive is an electronic device comprising a storage medium used to store information. As an example, a typical hard disk drive comprises a magnetic disk having concentric tracks capable of storing data. The magnetic disk is placed on a spin motor so that the disk can be rotated, and data stored on the disk is accessed by a head mounted on an actuator driven by a voice coil motor (VCM). The VCM rotates the actuator with a VCM driving current, and as a result, the head moves across the disk to access data.
The head reads information recorded on the surface of the disk by sensing changes in a magnetic field apparent on the surface of the disk, and the head stores information on the surface of the disk by generating a magnetic field in response to a write current supplied to the head. The magnetic field generated by the head causes the disk surface to be magnetized so that data is recorded or stored on the disk.
When the head reads information recorded on the surface of the disk, the magnetic field apparent on the surface of the disk produces a signal through the head. The signal is sampled through a sampling process and then converted into output data through a Viterbi decoding process.
Unfortunately, the accuracy of the output data tends to be very sensitive to the timing of the sampling process. As a result, variation in the timing of the sampling process tends to cause errors in the output data.
The timing of the sampling process is generally controlled by a sampling clock signal output by a voltage controlled oscillator (VCO), where the frequency of the sampling clock signal is derived from an oscillation frequency of the VCO. To stabilize the timing of the sampling process, the VCO relies on timing information recovered from the analog signal produced through the head to control its oscillation frequency.
In order to stably control the timing of the sampling process using conventional techniques, the tracking time required to compensate a timing offset (e.g., phase and/or frequency offsets) must be small, and the range of the timing offset that can be compensated, i.e., a pull-in range, must be wide.
Unfortunately, however, as the density of the recording medium increases, the signal-to-noise ratio of signals produced by the head of the hard disk drive tends to increase, and therefore it becomes increasingly difficult for conventional techniques to recover data encoded in those signals.